1. Area of the Art
This invention involves the application of microwave technologies to the assaying and extraction of metals such as gold and silver from their ores.
2. Description of the Background Art
Microwave energy may be useful in several ways in enhancing the extraction and recovery of trace metals from its ores. The basis of exploiting microwaves for extraction lies in the fact that microwaves are more readily absorbed by some minerals than others, leading to a preferential heating of such minerals. This preferential heating may result in different effects to those obtained by application of conventional thermal energy. Mineralogy can be expected to play an important part in the application of microwave energy to the treatment of gold ores because some minerals absorb microwave energy and become hot, and may decompose or react with oxygen or other substances. Other minerals however are ‘transparent’ to microwave energy and hence unaffected.
Nevertheless, minerals which are transparent at ambient temperatures may absorb microwave energy at higher temperatures. Gold is considered to be refractory when it cannot be easily recovered by alkaline cyanide leaching. The vast majority of refractory gold occurs in chalcogenide minerals such as pyrite (FeS2), arsenopyrite (FeAsS) and pyrrhotite (FeS). Generally, refractory gold concentrate or ore is pretreated by roasting, oxygen pressure leaching or bacterial leaching, to render it amenable to gold recovery by alkaline cyanide leaching. Because chalcogenide minerals are in general heated easily by microwaves, it is possible to pre-treat chalcogenide refractory gold concentrates by microwave energy. However, conventional microwave energy does not seem adequate.
While the application of microwave energy in technology has been explored worldwide for five decades as a way of saving energy, it has not come into use for any high temperature (>≈600° C.) applications. Over the past twenty years research the senior inventor and his group has made a major contribution to understanding and application of microwave energy in various fields of materials science [1-20].
The fundamental conceptual error in all previous research has been the mistaken belief that microwaves provide only an alternative source of thermal energy: i.e. that a material heated to say 800° C. in a resistance electric or gas-fired furnace is affected in an identical manner to one heated by microwaves. This view has now been superseded [18, 19].
In 2002 in a paper [21] and patent [22, U.S. Pat. No. 6,365,885, issued Apr. 2, 2002] issued to one of the current inventors, it was shown that the E (electrical) and H (magnetic) fields in a 2.45 GHz microwave reactor interact very differently with matter. For the first time it was demonstrated that the microwave field itself, independent of temperature, profoundly affected the thermodynamics of any system where 3d and 4f electrons have unpaired spins. This important effect is realized with single mode microwave radiation where the point of maximal E field and H field are spatially separated.
This hitherto unknown phenomenon is what we utilize in our present invention—the change of the stability of a phase such as a gold or silver bearing mineral by the E and H fields of the microwave radiation. In many cases the resulting phase change causes the stabilization of a new phase assemblage, most of which include the separation from the ore of a phase of the metallic element.